1 Technical Field of the Invention
The present invention relates to a valve timing adjusting apparatus for an internal combustion engine, which adjusts valve timing of at least one of intake and exhaust valves of the engine that are opened and closed by a camshaft driven by torque transmitted from a crankshaft of the engine.
2 Description of the Related Art
There have been known valve timing adjusting apparatuses for internal combustion engines, which adjust, with a phase adjusting mechanism connected to a motor shaft, the relative phase of a camshaft to a crankshaft according to the rotational condition of the motor shaft.
For example, Japanese Patent First Publication No. 2004-350446 discloses such a valve timing adjusting apparatus. More specifically, in the apparatus, there are provided a plurality of switching elements that are connected to stator windings of a motor; the stator windings create a magnetic field upon being energized. The apparatus drives a motor shaft, on which the created magnetic field acts, by shifting those of the switching elements which are turned on according to predetermined rotation angle ranges of the motor shaft.
However, in the above apparatus, induced voltage will be generated in the stator windings when the motor shaft rotates in the magnetic field created by the stator windings Further, when a target rotational direction of the motor shaft, which determines the shift sequence of the turned-on switching elements, is coincident with the actual rotational direction of the motor shaft/the direction of the induced voltage will be opposite to that of the voltage which is applied to the stator windings upon turning on those switching elements. Consequently, current corresponding to the difference between the applied and induced voltages will flow through the turned-on switching elements.
On the other hand, when the target rotational direction of the motor shaft is opposite to the actual rotational direction, the direction of the induced voltage will be coincident with that of the voltage applied to the stator windings. Consequently, large current corresponding to the sum of the applied and induced voltages will flow through the turned-on switching elements; this may cause the turned-on switching elements to be damaged due to overheating. Further, since the shift of the turned-on switching elements is made according to the rotation angle ranges of the motor shaft, the turned-on switching elements become more easily damaged as the rotational speed of the motor shaft increases.